Mycoplasma gallisepticum (MG) is an infectious respiratory pathogen of chickens and turkeys. It is the most pathogenic and economically significant mycoplasma pathogen of poultry. Economic losses from condemnation or downgrading of carcasses, reduced feed and egg production efficiency, and increased medication costs are factors that make this one of the costliest disease problems confronting commercial poultry production worldwide. The control of MG is generally by isolation and maintenance of breeding stock free of MG. However, the rapid expansion of poultry production in small geographic areas and multiple-age farms that never depopulate make the eradication and control of MG by such biosecurity efforts alone difficult and necessitate the implementation of additional measures. In these situations, prophylactic immunization of fowl against MG related disease involves the use of inactivated vaccines or exposure to live attenuated vaccine strains of MG. See, for example, Kleven et al., 1997, Acta Vet Hung; 45(3):299-305.
However, each approach has disadvantages. Inactivated vaccines, while generally effective in protecting against loss of egg production in layers, do not reliably prevent infection or provide consistent protection against respiratory disease. And, while live attenuated MG vaccines appear to be more effective, and therefore more popular, than inactivated vaccines, they can produce disease or impair reproductive function.
An important characteristic of an effective MG live vaccine is the ability to increase resistance to wild-type strain infection, and to displace wild-type strains with the vaccine strain on multiple-age production sites (Levisohn and Kleven, 2000, Rev Sci Tech; 19(2):425-42; and Turner and Kleven, 1998, Avian Dis; 42(2):404-7). Currently, live vaccines available for the control of MG include F strain (Luginbuhl et al., 1967, Ann NY Acad Sci; 143:234-238; Adler et al., 1960, Am J Vet Res; 21:482-485), 6/85 (Evans and Hafez, 1992, Avian Dis; 36:197-201) and ts-11 (Whithear et al., 1990, Aust Vet J; 67:159-165; and Whithear et al., 1990, Aust Vet J; 67:168-174). F strain is transmissible to unvaccinated pen mates and chickens in adjacent pens and can be isolated from farms long after vaccination has ceased. The strains ts-11 and 6/85 are transmissible, albeit poorly, from vaccinated to unvaccinated poultry when in contact. F strain persists at higher levels in the upper respiratory tract than either ts-11 or 6/85, and ts-11 appears to colonize more effectively than 6/85. F-strain also transmits from hen to egg. Unfortunately, although each of the currently available vaccines has its advantages, none of them attains the ideal status in every respect. Thus, there is therefore a need for improved live MG vaccine strains that are both safe and efficacious, that are stable and non-virulent.